模具方面的外文翻译.docx
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模具方面的外文翻译.docx
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模具方面的外文翻译
外文资料翻译:
PLASTICPRODUCTFAILUREDUETODESIGN,MATERIALORPROCESSINGPROBLEMS
ByMyerEzrin,GaryLavigneandJohnHelwigUniversityofConnecticut,InstituteofMaterialsScience
Abstract
Severalexamplesaregiveninwhichdesign,processing,oranaspectofthematerialwereprimarycontributorstofailureofplasticproducts.Acommonpatternisfailuretorealizetheconsequencesofseeminglyinconsequentialpracticesordecisions.Molddesignwasafactorinsomecases.Materialfactorsandprocessingwereinvolvedinothercases.Frequentlydesign,materialandprocessingaresocloselyrelatedthatfailurecannotbeascribedsolelytooneofthethree(8).
1.Introduction
Inmanycasesoffailurethecauseisatleastpartlyduetofailuretoknoworrealizethepotentialconsequencesofseeminglysafepracticesordecisions.Inmanyofthecasescitedfailureoccursatthemanufacturingstage,eitherinprimaryprocessing,suchasinjectionmolding,orinsecondaryoperations.Allfailurescanbetracedtothedesign,thematerial,orprocessing,assumingserviceconditionsarenotunusuallysevere.Theinterdependenceofthethreemaincausesoffailureissuchthatoftenallarecontributors.Materialandprocessingareparticularlystronglylinked.Thematerialcontributiontofailuremaybeinthepolymeritselforinanadditive.Processingimposesonplasticsthermalandmechanicalstressesthatfrequentlyarethemostsevereapartwillexperienceinitsentirelifetime.Failureisoftenduetolackofrealizationofhowseverethestressesinprocessingareandoftheeffectonthematerial.Examplesaregivenoffailuresduetopartdesign,molddesign,materialselectionandprocessing.
2.PartDesign
2.1Polypropylene(PP)capsforapackagingapplicationrequiredthatthetopofthecapbeflexedsubstantiallyduetodirectcontactwitharoundballatthetopofthecontainer.Fractureoccurredwithsomecapsfromthehighflexuralloadanddeformation.Thegatewasatthecenterofthetopofthecapwherestresswasgreatestinservice.Thedesignandmaterialcanwithstandtheservicestressesonlyifthematerialpropertiesareincontrol,whichwasnotthecase.Inadequateantioxidantandregrindusewerethemaincausesofmolecularweightbeingoutofcontrol.Thiscaseillustratesafailuretorealizehowreadilycertainpolymers,inparticularPP,degradeduringprocessingandthatasmallreductioninmolecularweight(MW)maybesufficienttocausefailure.Thedesignplayedapartinthatthefractureinitiationisatthegatewhichisinherentlyweak.Theeffectofprocessingonthematerialcanbemonitoredbyhowmuchmeltindexormeltflowrateincreasesinprocessing.Generallyanincreaseofmorethan10-20%inmostcasesmaybetoomuch,unlessthepartexperiencesverylittlestressinservice.ThecorrespondingdecreaseinMWmaybeonlyabout5%,yetthatmaybemorethanthedesignandtheservicestresseswilltolerate.Fortunately,meltindexisaconvenientandsensitivetestwhichtakesadvantageofthefactthatmeltviscosityisafunctionofthe3.4powerofMWaboveabout20,000MW(ç=KM3.4).Anotherrelativelysimpletestthatprovidesameasure,ineffect,ofantioxidantcontentisoxidativeinductiontimebydifferentialscanningcalorimetry(DSC)(ASTMD3895).Thistestisparticularlyapplicabletopolyolefins(PE,PP).Withoutadequateantioxidant,PPandPEareverysusceptibletooxidativedegradationduringprocessing.Whilethiscaseiscitedasanexampleofpartdesign,italsoillustrateshowmaterialandprocessingconsiderationsarealsoinvolved.
PresentedatNationalManufacturingWeek,DesignforManufacturabilityofPlasticParts,March16,1999,Chicago.
PLASTICPRODUCTFAILUREDUETODESIGN,MATERIALORPROCESSINGPROBLEMS
byMyerEzrin,GaryLavigneandJohnHelwig
2.2AnOringmadeofplasticizedPVCwasincontactwithapolycarbonatepartinanassemblythatrequired
thatthePCmovefreelywhentheOringpressurewasremoved.Inservicetherewassticking,i.e.,separationdidnotoccurreadilyasdesigned.PlasticizeratthesurfacetransferredtothePC,whichisnotcompletelyimpervioustoplasticizer.Ineffect,theplasticizerbecameanadhesivebetweenPCandPVC.ThisfailurewasprobablyalsodueinparttothefactthatplasticizersarelesscompatibleinPVCunderpressure.InthiscasetheeffectofplasticizeronPC,anamorphouspolymer,wasnotrealized,aswellasthepressureeffectoncompatibility.ABSisalsoadverselyaffectedbycontactwithplasticizerfromPVC.
2.3Bottlecapswerespraypaintedforcolorandscratchresistance.Thebottomofthecapsweretobebondedtoanotherpartofthecapwithsiliconeadhesive.Theadhesivefailedtobondtotheplasticinsomecases.Thecausewasthatsomespraypaintcontactedthebottomsurface.Waxyingredientsinthepaint,forscratchresistance,interferedwiththebondthatnormallywouldhavebeenmadetothesilicone.Thedesignandprocessingdidnottakeintoaccounttheneedtoprotectthebottomsurfacewhilethecapabovewasspraypainted.Itshouldhavebeenrealizedthateventracesofcontaminantonasurfacecanreducebondstrengthverystrongly.
3.MoldDesign
3.1AnABSinjectionmoldedpartofasyringeneedleholder(4)consistedoftwoflatsontheinside180E
apart.Ametaleyeletandtubinginsertedaftermoldingwereheldinplacebystressattheflats.Thedesigncalledfortheflats,whicharehighstresspoints,tobe90Eremovedfromthepart’stwoweldlines.Insomemoldcavitiestheflatswerenotlocatedasintended,sothattheflatswereattheweldlines,contributingtofailure.
3.2AhollowABSinjectionmoldedparthadatopringofABSultrasonicallyweldedintotheinside
diameterofthepart.Someweldshadaprotrusionatonepointinthecircumference,whichwasthoughttobeflashfromthewelding.Thesedefectsoccurredwithpartsfromoneofatwocavitymold.Loweringtheforceofinsertionofthetopringduringweldingdidnoteliminatealldefects.Examinationofmoldedpartsforfrozen-instressbyimmersioninaceticacid(ASTMD1939)showedverylittlestress.Acheckforoutofroundnessshowedthatbadpartswereoutofroundasmuchas±0.0025",comparedto±0.0005"forgoodparts.Figure1isacross-sectionalviewofaweldedjunctionobtainedbysandingdownaweldedunit.Thefailureisafractureoftheouterwallofthemoldedpart,whichoccurredonlywithoutofroundparts.Figure2isasketchofhowgoodandbadpartsfittogetherwiththeinsert.Fracturewasduetoflashpushingtheedgeofthepartoutwardsastheringinsertwasforceddown.Ingoodweldsalltheflashmoveddownwardinsidethepart.Inthiscasethehumanfailurewasnottocheckifpartsorthemoldcavitywereperfectlyround.
4.Material
4.1Aglass-filledPBT(polybutyleneterephthalate)parthadaholeinthecenterinwhichathreadedmetal
partmovedfreelybackandforth.Inovenagingat160ECtosimulateunderthehoodautomotiveservicethemetalpartlostitsabilitytomovefreelyinthePBTpart,whichhadshrunkenslightly.Shrinkagewasduemainlytofurthercrystallizationinservicebeyondthedegreeofcrystallinityasmolded.DSCshowedthattheheatoffusionincreasedapproximately20%,correspondingtoalikeincreaseindegreeofcrystallinity.Thecrystallinitydevelopedonagingat160ECisseenasanewpeakatapproximately200EC.Shrinkagewouldnotoccurifthepartwasfullycrystallized.Itwouldnotbeaproblemifthefitortolerancebetweenmetalandplasticwasnotsotight.PossiblyanucleatingagentinthePBTwouldgivecompletecrystallizationasmolded,sothatshrinkageasmoldedwouldnotoccurinservice.Whatwasnotrealizedwasthatcrystallinepolymersmayshrinkinserviceifnotfullycrystallized.
4.2Aprototypepartwasmachinedfromablockofplasticbelievedtobeacetalhomopolymer.Itperformedintrialrunsinservicebelowexpectations.Considerationwasbeinggiventoredesignortoachangein
PLASTICPRODUCTFAILUREDUETODESIGN,MATERIALORPROCESSINGPROBLEMS
byMyerEzrin,GaryLavigneandJohnHelwig
material.AcheckofthematerialbyinfraredspectroscopyandDSCshowedthatitwasHDPE,notacetal.ThetrialrunresultswereconsistentwithwhatwouldbeexpectedofHDPE.Thefailurewasinassumingincorrectlywhatthetypeofmaterialwas.
5.Processing
AsindicatedintheIntroduction,acommonfailureisnottorealizethatthemostsevereandpotentiallydamagingstageinaplastic’sentireexperienceisthethermalandmechanicalstressesofprocessing.Thisproblemisparticularlyseriousforcondensationpolymers(nylon,PET,PC,PUR)andforpolyolefins,althoughitisaproblemforallmaterials.Intheformercase,hydrolysistolowerMWcantakeplaceifwatercontentisaboveabout0.01%.Therequirementofpracticallycompletedrynessinthemeltcannotbeoveremphasized.ForpolyolefinslikePEandPP,oxygenistheenemy,togetherwithfreeradicals(reactivecarbonatomslackingonehydrogenatom)(5).Withoutadequateandeffectiveantioxidant,thestageforfailureissetinthemoldingmachineorextruder.Section2.1abovereferstothe3.4powerrelationshipbetweenmeltviscosityandMW.ForevenasmallincreaseinMW,themeltviscositypenaltyishigh,makingprocessabilityevenmoredifficult.Atthesametime,thecurveofproperties(strength,toughness,chemicalresistance)vs.MWislevelingoff(6),sothatthegaininproductperformancemaycomeatahighpriceinprocessability.ThelessonseemstobetodesignsothatpropertiescanbeachievedatthelowestMWpossible(highestMI).Theinterdependenceofdesign,materialandprocessingisveryhigh.Thisisnotedinsome
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